Our research uses the phenomenon of suppression of chain termination codons by mutant tRNA as a tool to detect the functional state of tRNA. We have established mutant strains of both yeast and E. coli in which a mutation outside the tRNA structural gene alters the function of the tRNA. We are mapping, characterizing and defining the physiological and biochemical changes associated with such "tRNA modification mutants." Our results to date have enumerated the genetic loci of several mutations, and have demonstrated that a single defined yeast suppressor tRNA can be transposed to at least three apparently specific locations in the yeast genome. We have characterized the DNA fragments in phage T4 which carry tRNA genes and have defined several steps essential to maturation of precursor tRNA into functional tRNA. Our work in the coming year will elucidate the biochemical nature of the genetic lesion as well as better define the genetic behavior of suppressor tRNA and of modification mutants which result in loss of suppressor function. This basic data is essential to the understanding of genetic lesions in higher organisms.